This invention relates to an improved process for selectively etching semiconductor devices, and more particularly to a process for effectively and efficiently etching multi-layer semiconductor devices using an improved resist etching mask.
It is known in the prior art that the manufacture of multi-layer semiconductor devices, such as in the fabrication of very large scale integration (VLSI) semiconductor chips including stacked capacitor Dram memory chips, can be produced by patterned etching using liquid or wet etching materials of certain layers of these devices. Etching can also be conducted in a gas phase using known techniques such as plasma etching, ion beam etching, and reactive ion etching.
In the etching process of semiconductor devices, a protective etching mask is first formed employing a layer of photoresist material ("resist") disposed on a major surface of the semiconductor device. This protective resist etch mask is designed to facilitate the formation of a desired pattern of lines and spaces in the resist layer based on a predetermined etch pattern in the resist layer. Thus, when an optical source, such as UV light, contacts these predetermined resist areas, a pattern is created in the resist structure which exposes the requisite portions of the semiconductor surface for subsequent etching purposes. The higher the number of narrow lines, and the narrower the spaces therebetween, i.e., widths of three microns or less, the more the capacitance per unit area which can be stored on a given semiconductor device, and the higher the resolution of that device. Once the protective etching resist mask are in place on the semiconductor surface, etching of the exposed areas can commence to produce a semiconductor having a predetermined etched pattern.
One of the first resist materials used in semiconductor fabrication produced a negative image and thus was called negative resist. As shown in FIG. 1, areas where the optical source strikes becomes polymerized and more difficult to remove. When the resist is "developed" (subjected to a solvent), polymerized regions remain and the unpolymerized areas are removed. These negative resists have three components. First is the resin portion which is radiation insensitive but is extremely soluble in non-polar organic solvents. Polyvinyl cinnamate was used as the basic resin in most of the resists of the 1960's. However, most of these negative resist resins today are based on cyclized polyisoprene polymers. The second component is the sensitizer which is a photoactive compound. A photochemical reaction is initiated which generates a cross-linked three-dimensional molecular network that is insoluble in a developer material. The most important photoreaction is the evolution of nitrogen from the excited state of the arylazide to form an extremely reactive intermediate compound called a nitrene. A wide range of sensitizers may be used, including quinones, azido compounds, and nitro compounds for polyvinyl cinnamate and azides for cyclized polyisoprene. A small quantity of novolak resin can be added to improve adhesion during developing. The third component is the developer. Developers include nitrobenzene and furfural for polyvinyl cinnamate, and xylene and benzene for polyisoprene.
Historically, negative resists have been unsuitable for applications requiring line and space dimensions of the resist pattern which are less than 3 microns, see "Silicon Processing For The VLSI ERA", Volume 1-Process Technology, S. Wolf and R. N. Tauber (1987). The major problem is swelling. Regardless of the resin or additives, negative resists are known to suffer from swelling problems during development. Even though the developer doesn't dissolve the exposed resist, it is absorbed therein and causes swelling. During subsequent rinse operations, the developer is removed, and the resist shrinks. If the resist lines are closer together than 3 microns (as required for high resolution), the swelling can cause them to touch. During the subsequent shrinkage process, they may remain stuck together thereby eliminating the required spacing between lines. Long and narrow resist lines can also become wavy during swelling and, if surface adherence is good, will retain their waviness after shrinkage. Furthermore, the resist lines can be pulled loose from the substrate.
Because the above problems with negative resists, positive resists replaced the negative resist materials. Positive resists are based on a totally different chemistry than negative resists. As shown in FIG. 2, exposure to an optical source changes the positive resist material so that it is solubilized and can be more easily removed. Since the exposed regions are removed, the resist is referred to as positive. Positive resists function very differently from negative resists. The sensitizer and the resin do not interact, so the change in solubility is all due to the sensitizer. The sensitizer breaks down under the influence of an optical source and increases the solubility rate in alkaline solutions by a factor of about a thousand. Since the presence of the sensitizer inhibits dissolution, it is often referred to as an inhibitor. While negative resists use only 2% or 3% sensitizer, in positive resists 20% may be used. The most common sensitizer is naphthoquinone diazide. Ethylene glycol monomethyl can be employed as a solvent, while diluents may include butyl and cellusolve acetate. Positive resists have a broader optical sensitivity than negative resists and can utilize the output of a more of a conventional UV lamp. By exposing the resist to the desired optical pattern in a vacuum and then flood-exposing the entire surface with UV, the previously unexposed resist becomes soluble and that exposed in vacuum remains insoluble. In this way, a positive resist can produce a resist pattern with lines and spaces having a three micron or less dimension without the swelling problems associated with negative resists.
Sequential layering processes employed in the fabrication of stacked capacitor Dram memory chips and other VSLI semiconductor chips can result in situations where conductive films are needed to be deposited on surfaces with "re-entrant" profiles which have an irregular configuration. For instance, as seen in FIGS. 1 and 2, in the formation of TEOS oxide-polysilicon semiconductor devices 10, the outer portion of the outer surface of a topographic layer 12, typically silicon dioxide, is formed in a "winged" configuration. This winged configuration forms recesses or "caves" 14. A conductive layer 15, generally polysilicon, is formed on the outer surface of the topographical layer 12. Photolithographic patterning and etching is conducted on this device having a winged configuration. When a conventional positive resist material 16 is employed, such as a novolak resin resist in combination with oxygen plasmas or aggressive developers, residual resist material 16' is "trapped" in the caves 14 because the resist material is not exposed to UV light (see FIG. 2). The is because the UV light, denoted by arrows "20" in FIG. 1, is unable to penetrate into the caves. In turn, the conductive layer 15 located within the confines of the caves 14 is not subsequently removed during the etching process. Thus, the etching process becomes inhibited by underexposed and/or underdeveloped residual resist on sidewalls of the re-entrant profiles and out-of-focus deep troughs resulting in conductive shorts remaining after etching. Even after UV-exposure and developing residual resist material 16', typically in the form of a resist stringer, remains in the caves. An ancillary etching procedure must then be performed in which a basic pH develop or an oxygen-containing plasma is employed to remove the residual resist material and conductive layer. This results in a undesirable CD loss.
Therefore, a need exists for a process for etching semiconductor devices which can produce resist patterns having line and space dimensions which are less than 3 microns, which in turn produce semiconductor devices having high resolution etched patterns, without the formation of residual resist material and without requiring a subsequent residual resist removal process.